In recent years, with the development of small-size information equipment, rapid miniaturization of electronic components mounted thereon has been advanced. To the electronic components, a ball grid array (hereinafter referred to as “BGA”) in which electrodes are arranged on a back surface is applied for corresponding to narrowing of connection terminals and/or reduction of mounting area by requirement of the miniaturization of electronic components.
As the electronic components to which BGA is applied, a semiconductor package has been exemplified. In the semiconductor package, semiconductor chips having electrodes are sealed with any resin. A solder bump is formed on each of the electrodes of semiconductor chips. This solder bump is formed by joining the solder ball to the electrode of the semiconductor chip. The semiconductor package to which BGA is applied is mounted on a printed board so that the solder bumps are in contact with the conductive land on the printed board and the solder bumps melted by heating and the land are joined to each other. In addition, in order to cope with a requirement of further high-density packing, any three-dimensional high-density packings in each of which the semiconductor packages are stacked in a height direction thereof have been studied.
However, when BGA is applied to a semiconductor package having such three-dimensional high-density packings, the solder balls may be crushed by the self-weight of the semiconductor package, and short circuits occur between the electrodes. This may hinder high-density packings from being constituted.
Therefore, solder bumps utilizing balls each having a very small diameter and being made of metal such as Ni having a melting point higher than that of the solder have been studied. When electronic components are mounted on a printed board, the solder bumps including Ni balls or the like can support the semiconductor package by the Ni balls which do not melt at the melting point of the solder, even if the weight of the semiconductor package is applied to the bumps. Thus, the solder bumps are not crushed by the self-weight of the semiconductor package. As the related art, for example, Patent Document 1 is listed.
By the way, the miniaturization of electronic components has allowed the high-density packing, but the high-density packing has caused software error problems. Software errors are caused by being capable of rewriting the stored contents of the memory cells of a semiconductor integrated circuit (hereinafter referred to as “IC”) by enabling the α-ray to enter into them. It is conceivable that the α-ray radiates with α decay of radioactive isotopes such as U, Th, 210Po in the solder alloy. Therefore, in recent years, any low α-ray solder materials having a low radioactive isotopic content have been developed.
Therefore, the Ni balls contained in the solder as described in Patent Document 1 are also required to reduce software errors caused by the high-density packing.
Patent Document 1: Japanese Patent Application Publication No. 11-261210